Riven's Sword

Nah that doesn't make sense to me. If Riven felt betrayed AFTER the bomb why would she still fight unless it was maybe in self-defense but even in that situation Souma was just chillin' in his dojo or something the new lore seems to imply they never met and someone ELSE killed Souma. To me it sounds like the potential for a new character, champion or lore piece. It could be many people too, some random Noxian, a jealous student that wanted to train under him, or perhaps Yasuo's father for some reason or Riven's master after the betrayal. Who knows shrug :/ (Rito knows).

Well...why she would do that? She felt so broken, betrayed and sad for what heppened, killing the elder will do more damage to Ionia, making things worse Also i don't see her doing that before the explosion, this lore makes her more "clean"

It Yasuo. He broke the wind right before he left. If he didn't immediately let it rip, his master woulda been clear.